What are known as prepreg systems are conventionally produced in an autoclave, from fibres which have been pre-impregnated with a matrix material, the matrix material of a formed prepreg semi-finished product undergoing a curing process under the influence of negative pressure and temperature. However, this method is not suitable for carrying out in situ repairs, for example on aircraft, since it is obviously not possible to accommodate the entire aircraft in an autoclave. Repairs of this type therefore have to be carried out by a different method outside an autoclave. Processes of this type, known as “out-of-autoclave” processes, are known in the art. However, the mechanical properties and in particular the strength of composite moulded parts of this type, which are produced outside an autoclave, are considerably worse than for moulded parts which have been cured in an autoclave.
From the scientific publication “Out-of-autoclave processable prepregs and resin films: An overview of recent developments and shared database”, Ridgard C., SAE technical paper series, No. 2006-01-3164, it is known to cure composite components of prepreg material in negative pressure heating bags, for example so as to produce prototypes. The strength of composite components which are produced in this manner does not correspond to the strength of components which are baked in an autoclave, but is often sufficient for prototype construction. Meanwhile, it is possible to produce prepreg materials which are close, in terms of fibre density and porosity, to prepreg materials which are cured in an autoclave, but the mechanical properties thereof are worse than for prepreg materials which are cured in the furnace, in particular when loaded with pressure or impacts, since for example a different plastics material is used as the matrix material.
A drawback of what are known as out-of-autoclave prepreg systems is that they have to have a low-viscosity matrix material so as to achieve a low porosity and a high fibre content by volume. This has the result that the mechanical strength of the products produced therefrom is lower than for products which are cured in an autoclave, in such a way that the out-of-autoclave process is not suitable for repair purposes in which a high mechanical loading capacity has to be achieved.
In parallel with the development of out-of-autoclave prepregs, novel methods for introducing matrix material into dry fibre material have also been developed. One of these novel methods is what is known as the “vacuum-assisted process” (VAP), which is known for example from the literature source “Principles of the vacuum assisted process and its application for aerospace components”, Körwien T., ISCM 06 (“3rd International Symposium on Composites Manufacturing Technology for Aircraft Structures”, 17 to 18 May 2006). In this method of the vacuum-assisted process, a dry fibre material is covered with a gas-permeable microporous membrane, which forms a barrier for the matrix material consisting of synthetic resin. As a result of applying a negative pressure, the matrix material is sucked out from a storage container into the dry fibre material. A special variant of the VAP method is disclosed in EP 1 393 883 A1, in which the aeration space, between the membrane and the outer vacuum sack, and the injection space, between the component to be produced and the membrane, can each be evacuated separately.
Another improvement on the curing process for prepregs is what is known as the double vacuum bag (DVB) method, which is described for example in WO 2005/113213 A2 or the literature “NASA LAR-16877, Double-vacuum-bag process for making resin-matrix composites”. In this method, a composite moulded part blank, of a prepreg material consisting of fibres which are pre-impregnated with a matrix material, is laid between two metal moulding plates, and this arrangement is subsequently enclosed in a vacuum bag, which forms an inner bag which is fixed in a sealed manner on one of the moulding plates. The inside of this inner bag is connected to a vacuum pump. A tool in the form of an inverted perforated beaker is placed above this arrangement. Outside this bag, a further vacuum bag is placed around the entire arrangement as an outer vacuum bag. The inside of this outer vacuum bag is also attached to a vacuum pump. This entire arrangement is subsequently placed in a hot air furnace and subjected to a predetermined curing process. In this context, a negative pressure is initially applied to the outer bag, in such a way that it lies against the outside of the beaker-shaped structure, resulting in a negative pressure in the remaining space outside the inner bag. Subsequently, a smaller negative pressure is applied to the inside of the inner bag. In this context, the stronger negative pressure surrounding the inner bag prevents the inner bag from collapsing onto the moulded part which is to be cured. The negative pressure which is present in the inside of the inner bag ensures that gases which are produced during the curing process escape from the prepreg material, and are not included in the material as gas bubbles during the curing. After a predetermined period, the inside of the outer bag is exposed to the ambient pressure again, and as a result, the inner bag collapses onto the moulded part and mechanically compresses it. This is followed by a further curing process for a predetermined time at a higher temperature. A drawback of this method is the complicated construction resulting from the beaker-shaped support structure for the outer vacuum bag.
The double vacuum bag method DVB does not achieve the high mechanical strength which is exhibited by prepregs which have been cured in an autoclave.
DE 10 2008 006 261 B3 discloses a multilayer, flexible sheet material which comprises a gas-permeable membrane and a textile layer, which is laminated onto said membrane and forms a gas-conducting structure. A gas-impermeable layer may additionally be applied on the side of the textile layer remote from the membrane.
DE 10 2008 015 070 B3 discloses a method for producing fibre composite components, which is specifically designed for application in an autoclave. For this purpose, there is an inner component space inside an outer vacuum space, the two spaces being interconnected in such a way that they can be evacuated together. As the external pressure is increased during the curing phase of the component, the connection between the two spaces closes automatically, in such a way that no infusion resin can escape from the component space.